1. Field of the Invention
The present invention relates generally to frequency-modulated (FM) radar systems for detecting the position of an object or target, and more particularly to such an FM radar system which is particularly suitable for the detection of the position of a target moving at high speeds.
2. Description of the Related Art
FM radar systems are known, which are designed to transmit a frequency modulated (FM) signal while receiving a signal reflected by the object so as to detect an object or target based on a beat signal obtained by mixing the received signal and a signal related to the transmitted signal. The known FM radar systems are disclosed, for example, in Japanese Patent Laid-open Publications Nos. HEI 4-142486, HEI 5-150035, HEI 5-180931, HEI 6-242230, HEI 7-128440 and HEI 8-211145.
In the FM radar system disclosed in Japanese Patent Laid-open Publication No. HEI 5-180931, a beat signal is detected after amplification by an amplifier whose amplification factor varies in proportion to the frequency. The amplifier may have a gain varying as the fourth power of the frequency. This publication also discloses a method in which a direction to the target is detected from the F-.theta. characteristic on the basis of a spectrum level of the detected signal.
In cases where the target is stationary or fixed in position relative to the FM radar system, the frequency of the beat signal has a value corresponding to a range or distance to the target (i.e., a value proportional to a relative distance between the radar system and the target). However, in the case where there is a relative velocity between the target and a movable body, such as a vehicle or an airplane on which the FM radar system is mounted, and particularly when the target is moving at high speeds relative to the FM radar system, the frequency of the beat signal tends to deviate to some extent from a value corresponding to the real distance (relative distance) to the target due to the Doppler shift or the change in observed frequency of a received signal.
In the case where the beat signal is set to be amplified with a particular amplification factor varying with the frequency, the Doppler shift causes the amplification of the beat signal to be actually executed with a different amplification factor which does not correspond to a real frequency. The amplified signal eventually has a spectrum level somewhat deviated from the real spectrum level.
Especially, in the case where the amplification is set to be taken with an amplification factor varying to such a greater extent as equal the fourth power of the frequency, and when the target is moving at high speeds, the Doppler shift causes a frequency shifting which will make the amplification take place with an amplification factor proportional to the fourth power of the deviated frequency. As a result, the spectrum level of the amplified signal becomes greatly deviated from the real spectrum level. Under these circumstances, the direction to the target, which is detected on the basis of the spectrum level of the amplified signal, is greatly deviated from the real direction.
The FM radar system disclosed in Japanese Patent Laid-open Publication No. HEI 7-128440 includes: a frequency analyzing means or analyzer for analyzing the frequency of a beat signal to obtain a spectral distribution at every unit frequency interval of the beat signal; a maximum spectrum detecting means or detector for determining from the spectral distribution, the frequency of a maximum spectrum at which the spectrum level takes a maximum value above a predetermined level; a level centroid detecting means or detector for determining the frequency at the level centroid of the spectral distribution within a predetermined frequency width through the front and the rear of the maximum spectrum; and a distance detecting means or detector for determining a relative distance of a target on the basis of the frequency of the level centroid which is a frequency corresponding to the relative distance of the object.
Detection of the maximum spectrum is accomplished by detecting a spectrum which has a spectrum level larger than the predetermined level and which is located between two spectrums at adjacent frequency bands whose spectrum levels each indicate a change from the upward inclination to the downward inclination when viewed from the central spectrum.
Japanese Patent Laid-open Publication No. HEI 7-128440 also shows a modified radar system wherein data on the relative distance of a target and data on the radiating direction of a beam are paired to determine the position of a two-dimensional level centroid, and the determined two-dimensional level centroid position is taken to be a two-dimensional relative position of the target.
The radar systems disclosed in Japanese Patent Laid-open Publication No. HEI 7-128440 operate generally satisfactorily, however, there is a limit in ability to detect the position of the target with high accuracy because the effect of the Doppler shift is not taken into consideration.